start_secondary(), do_double_fault(), play_dead(), and tboot_s3_error()
never return, so would better be annotated anyway.

context_switch() and continue_running() are odd: We can't
(unconditionally) add noreturn to their declarations, as Arm's variants do
return. Put the attribute on x86'es definitions instead (the use of
unreachable() in reset_stack_and_call_ind() allows the compiler to figure
that out itself, but Eclair wants the annotation in addition).

Note that adding "noreturn" to do_double_fault() causes new violations of
the same rule (2.1) at the two call sites in entry_from_{pv,xen}().

Signed-off-by: Jan Beulich <jbeulich@suse.com>
Reviewed-by: Nicola Vetrini <nicola.vetrini@bugseng.com>
---
https://gitlab.com/xen-project/hardware/xen-staging/-/pipelines/2916849077

entry_from_pv() wants the annotation only when PV=n, yet once added gcc
then warns about "return" being used in a "noreturn" function. Is there
any other approach to address this besides adding #ifdef inside the
function (i.e. replacing the !IS_ENABLED(CONFIG_PV) check that's there)?
Perhaps need to deviate it?
---
v2: Drop entry_from_{pv,xen}() changes, for being controversial.

--- a/xen/arch/x86/domain.c
+++ b/xen/arch/x86/domain.c
@@ -2163,7 +2163,7 @@ static void __context_switch(void)
     per_cpu(curr_vcpu, cpu) = n;
 }
 
-void context_switch(struct vcpu *prev, struct vcpu *next)
+void noreturn context_switch(struct vcpu *prev, struct vcpu *next)
 {
     unsigned int cpu = smp_processor_id();
     struct cpu_info *info = get_cpu_info();
@@ -2240,7 +2240,7 @@ void context_switch(struct vcpu *prev, s
     reset_stack_and_call_ind(nextd->arch.ctxt_switch->tail);
 }
 
-void continue_running(struct vcpu *same)
+void noreturn continue_running(struct vcpu *same)
 {
     reset_stack_and_call_ind(same->domain->arch.ctxt_switch->tail);
 }
--- a/xen/arch/x86/include/asm/cpuidle.h
+++ b/xen/arch/x86/include/asm/cpuidle.h
@@ -26,7 +26,7 @@ static inline int mwait_idle_init(struct
 int cpuidle_init_cpu(unsigned int cpu);
 void cf_check default_dead_idle(void);
 void cf_check acpi_dead_idle(void);
-void play_dead(void);
+void noreturn play_dead(void);
 void trace_exit_reason(u32 *irq_traced);
 void update_idle_stats(struct acpi_processor_power *power,
                        struct acpi_processor_cx *cx,
--- a/xen/arch/x86/include/asm/tboot.h
+++ b/xen/arch/x86/include/asm/tboot.h
@@ -126,7 +126,7 @@ int tboot_in_measured_env(void);
 int tboot_protect_mem_regions(void);
 int cf_check tboot_parse_dmar_table(acpi_table_handler dmar_handler);
 int tboot_s3_resume(void);
-void tboot_s3_error(int error);
+void noreturn tboot_s3_error(int error);
 int tboot_wake_ap(int apicid, unsigned long sipi_vec);
 #else
 static inline void tboot_probe(void) {}
--- a/xen/arch/x86/smpboot.c
+++ b/xen/arch/x86/smpboot.c
@@ -326,7 +326,7 @@ static void set_cpu_sibling_map(unsigned
     }
 }
 
-void asmlinkage start_secondary(void)
+void asmlinkage noreturn start_secondary(void)
 {
     struct cpu_info *info = get_cpu_info();
     unsigned int cpu = smp_processor_id();
--- a/xen/arch/x86/traps.c
+++ b/xen/arch/x86/traps.c
@@ -1080,7 +1080,7 @@ const char *vector_name(unsigned int vec
     return (vec < ARRAY_SIZE(names) && names[vec][0]) ? names[vec] : "???";
 }
 
-void asmlinkage do_double_fault(struct cpu_user_regs *regs)
+void asmlinkage noreturn do_double_fault(struct cpu_user_regs *regs)
 {
     unsigned int cpu;
     struct extra_state state;